1. Field of the Invention
The present invention relates to a fixing device for heating, pressurizing and fixing a developer image on recording media using fixing belt heated by a heating roller.
2. Description of the Related Art
In recent years, out of fixing devices that are used in electro-photographic type copiers and printers for heating and pressure fixing toner images formed on a sheet of paper, a belt type fixing device has been developed. This fixing device is excellent in keeping an uniform fixing temperature and capable of obtaining a good fixing even for an image with toners superposed like a full-color image.
This belt type fixing device has an endless belt type fixing belt that is put over between a fixing roller and a heating roller and heated by a heating roller. A pressing roller is brought in contact with the fixing belt supported by the fixing roller and presses the belt, and a sheet of paper is forced to pass through a nip formed between the fixing belt and the pressing roller. A toner image formed on a sheet paper is heated and press fixed.
In such a belt type fixing device, when the heating roller (or the fixing belt on the heating roller) was controlled at the same temperature in the ready state and in the fixing operation, the fixing belt was not heated sufficiently when the next fixing operation was started and the defective fixing was caused by the low temperature offset phenomenon. This defective fixing is generated by a temperature drop for heat radiation of the fixing belt at the part not in contact with the heating roller. On the other hand, when the fixing is performed after the fixing belt is heated to a prescribed temperature when starting the fixing operation as a countermeasure to prevent this low temperature offset phenomenon, there will be generated such a problem that a first copy time becomes long.
In order to solve such a problem, an apparatus to set the heating roller temperature in the ready state (or the fixing belt temperature on the heating roller) at a higher temperature than the heating roller temperature in the fixing operation has been developed. Thus, by setting the temperature of the heating roller, the temperature rise of the fixing belt at the time of starting the fixing was made fast and the first copy time was prevented from becoming long and generation of the low temperature offset phenomenon was also prevented.
However, when the heating roller was so controlled that its set temperature became higher in the ready state than that in the fixing operation as in a conventional apparatus, if the next fixing operation was started immediately after shifting to the ready state when completing the fixing operation, the temperature of the fixing belt at the time when starting the next fixing operation became to higher than a prescribed fixing temperature, the high temperature offset phenomenon was generated and the fixing became defective. This problem was caused as the fixing belt was further heated at a higher temperature in the state wherein its temperature was not sufficiently dropped by the heat radiation.
Accordingly, in the belt type fixing device, even when the next fixing operation was started immediately after completing the current fixing operation, it is desirable to prevent the generation of the high temperature offset phenomenon resulting from the heating of the fixing belt to above a prescribed fixing temperature and obtain fixed images of high quality without badly affecting reduction of a first copy time.
An object of the present invention is to obtain fixed images in a belt type fixing device without impairing reduction of a first copy time by preventing generation of the high temperature offset phenomenon when making the next the fixing operation immediately after completing the current fixing operation.
According to the embodiment of the present invention, there is provided a fixing device comprising: an endless belt shaped first heating device for heating and fixing a developer image formed on the recording medium by contacting it; plural supporting members for supporting the first heating device rotatably; a pressing device provided opposing to the supporting members via the first heating device for conveying the recording medium by clamping it jointly with the first heating device; a second heating device for heating the first heating device by keeping a first set temperature during the heating and fixing of the developer image by the first heating device and keeping a second set temperature higher than the first set temperature during the ready state; and a temperature controller for raising the temperature of the second heating device to the second set temperature from the first set temperature after the first heating device drops to a prescribed temperature lower than that during the heating and fixing when the fixing device is shifted to the ready state from the heating and fixing state by the first heating device.
Further, according to the present invention, there is provided a fixing device comprising: a fixing belt for contacting a recording medium, heating and fixing a developer image formed thereon; plural supporting rollers for supporting the fixing belt rotatably; a pressing roller provided opposing to the supporting rollers for clamping and conveying the recording medium jointly with the fixing belt; a heating roller for heating the fixing belt by keeping the set temperature during the heating and fixing the developer image by the fixing belt and keeping the second set temperature higher than the first set temperature during the ready state; and a temperature controller for raising the temperature of the heating roller to the second set temperature from the first set temperature after the fixing belt drops to a prescribed temperature lower than that during the heating and fixing when the fixing belt is shifted from the heating and fixing state to the ready state.
Further, according to the present invention, there is provided a temperature control method comprising the steps of a first step for keeping the fixing belt at the first set temperature during the heating and pressure fixing of the developer image; a second step for raising the temperature of the heating roller to the second set temperature from the first set temperature after the fixing belt drops to a prescribed temperature lower than that during the heating and fixing after completing the heating and pressure fixing of the developer image; and a third step for keeping the fixing belt at the second set temperature during the ready state.